As a secondary battery with a high capacity, a sodium-sulfur battery has been known. In the sodium-sulfur battery, molten sulfur makes a cathode active material, molten sodium makes an anode active material, solid-electrolyte beta-alumina, through which sodium ions Na+ permeate, makes a partition wall that isolates the molten sulfur and molten sodium from one another. Moreover, the molten salt, namely, a cathode active material, is accommodated in a cathode chamber; the molten sodium, namely, an anode active material, is accommodated in an anode chamber. The cathode chamber and anode chamber are connected electrically with the cathode terminal and anode terminal of the sodium-sulfur battery, respectively.
When discharging the sodium-sulfur battery, the sodium Na in the anode chamber separates into electrons and sodium ions Na+. The electrons flow from the anode terminal to the outside; the sodium ions Na+ permeate through the partition wall to move to the cathode chamber. In the cathode chamber, the cathode terminal donates electrons, the donated electrons, the sodium ions Na+, and molten sulfur S react chemically with each other to generate sodium polysulfide Na2Sx. When charging the sodium-sulfur battery, reactions, which reverse the reactions during the discharging operation, occur. During the charging operation, sodium ions Na+ and sulfur S generate from the sodium polysulfide Na2Sx, and then the sodium ions Na+ permeate through the partition wall to move from the cathode chamber to the anode chamber in the process. That is, the sodium ions Na+, which permeate through the partition wall, move from the anode chamber to the cathode chamber during the discharging operation, but move from the cathode chamber to the anode chamber during the charging operation. The sodium-ion battery operates at a high temperature falling in a range of from 290 to 350° C., because it is necessary that all of the molten sulfur and molten sodium in the sodium-ion battery, namely, the active materials, are in a molten state, namely, to be a liquid, respectively.
In the sodium-sulfur battery broken due to any cause, a large amount of the molten sodium makes contact with a large amount of the molten sulfur to react with one another so that the reaction generates heat in a large amount. Patent Literature No. 1 discloses a sodium-sulfur battery which is capable of preventing the molten sodium from flowing out in a large amount. The sodium-sulfur battery comprises a sodium container disposed independently of the anode chamber, accommodating most of the sodium, and made of a metal which is less likely to be fractured. Moreover, the sodium container is connected with the anode chamber by a long and fine communication tube functioning as a sodium passage.
In the process of the charging and discharging reactions, the sodium ions Na+ move from the cathode chamber to the anode chamber, or from the anode chamber to the cathode chamber, by way of the partition wall. Since the anode chamber communicates with the sodium container, the increase and decrease in the sodium amount within the anode chamber result in the decrease and increase in the sodium amount within the sodium container. That is, the sodium moves from the anode chamber to the sodium container so as to always fill up the anode chamber with the sodium. The anode chamber accommodates the sodium in a small amount. In short, the sodium container, which is made of a metal being less likely to be fractured, accommodates most of the sodium. Besides, the long and fine communication tube connects the sodium container with the anode chamber.
Thus, even if the partition wall demarcating the anode chamber should have been broken, the molten sodium is managed to flow out from the anode chamber in a small amount. Moreover, since the sodium container, which is made of a metal being less likely to be fractured, accommodates most of the molten sodium, and since the long and fine communication tube refrains the molten sodium from flowing out to the outside, the molten sodium is less likely to flow out to the outside. Accordingly, most of the molten sodium neither makes contact with the molten sulfur nor reacts with it. Consequently, even if the partition wall should have been broken, the broken partition is less likely to lead to the generation of heat in a large amount, and is unlikely to result in a fire. Note that the sodium-sulfur battery, which should have been broken to terminate the functions, involves a decrease in the temperature. The temperature decrease however turns both of the molten sodium and sodium polysulfide in the form of liquid into their stable solids, which surely makes chances of the reaction between sodium and sulfur disappear.